Superabsorbents are known, the term designating crosslinked hydrophilic polymers capable of imbibing large amounts of aqueous fluids. This capability rests on the strong interaction of water with hydrophilic groups on the superabsorbents, in particular ionic groups or groups capable of hydrogen bonding. Other customary designations for what are known as superabsorbents include “superabsorbent polymer”, “hydrogel” (often even used for the dry form), “hydrogen-forming polymer”, “water-absorbing polymer”, “absorbent gel-forming material”, “swellable resin”, “water-absorbing resin” or the like. Water-absorbing polymers based on partially neutralized acrylic acid are concerned in particular. The essential properties of superabsorbents are their ability to absorb a multiple (30-800 times for example) of their own weight of aqueous fluids and to retain the fluid even under some pressure. The superabsorbent, which is used in the form of a dry powder, turns into a gel on imbibing liquid, and so turns into a hydrogel when, as typical, imbibing water. Crosslinking is essential for synthetic superabsorbents and renders the polymers insoluble in water. Soluble substances would not be useful as superabsorbents. By far the most important field of use for superabsorbents is that of absorbing bodily fluids. Superabsorbents are used, for example, in diapers for infants, incontinence products for adults or femcare products. Fields of use further include, for example, as a water-retaining agent in market gardening, as a water storage medium for protection against fire, for fluid absorption in food packaging, as cable cladding material for deep sea cables or, very generally, for absorption of moisture.
Such a superabsorbent in general has a CRC (“Centrifuge Retention Capacity”) of at least 5 g/g, preferably at least 10 g/g, more preferably at least 20 g/g, especially 30 g/g. It is not just its absorption capacity which is important for a superabsorbent, but also its retention (ability to retain liquid under pressure usually expressed as “Absorption against Pressure” (“AAP”)) and also its permeability, i.e. the ability to conduct liquid in the swollen state. Flow conductivity to as yet unswollen superabsorbent may be blocked by swollen gel (“gel blocking”). Good conductivity properties for liquids are shown, for example, by hydrogels that have a high level of gel strength in the swollen state. Gels having only low gel strength are deformable under an applied pressure (body pressure), cause pores to clog in a superabsorbent/cellulose fiber pad and thereby block flow conductivity to as yet unswollen or incompletely swollen superabsorbent and the imbibition of liquid by this, as yet unswollen or incompletely swollen superabsorbent. Elevated gel strength is generally achieved through a relatively high level of crosslinking, but this reduces the absorption capacity of the product. A standard method of increasing gel strength is to increase the level of crosslinking at the surface of the superabsorbent particles compared to the interior of the particles. To this end, in a surface postcrosslinking step, dried superabsorbent particles having an average crosslinking density are subjected to additional crosslinking in a thin surface layer of the particles. Surface postcrosslinking increases the crosslink density in the shell of the superabsorbent particles, raising the absorption under confining pressure to a higher level. While the absorption capacity in the surface layer of the superabsorbent particles decreases, the presence of mobile chains of polymer in their core leads to an improved absorption capacity compared with the shell, so shell construction ensures an improved permeability without occurrence of gel blocking. It is likewise known to produce comparatively highly crosslinked superabsorbents overall and to subsequently reduce the degree of crosslinking in the interior of the particles versus an outer shell of the particles.
The manufacture of such superabsorbents (also called superabsorbent polymers) is based essentially on the polymerization of ethylenically unsaturated acid-functional monomers which are optionally at least partly present as a salt, in particular on the free-radical polymerization of partially neutralized acrylic acid, typically in the presence of crosslinkers. A free-radical polymerization reaction is a fast reaction and a strongly exothermic process.
This reaction leads to the construction of a three-dimensional polymeric network which may have different macroscopic properties, depending on process conditions and reaction procedure. When, for example, the polymerization reaction takes place within a very short time accompanied by very considerable evolution of heat, defects may develop in the three-dimensional polymeric network, for example as a consequence of chain transfer reactions, to have an adverse effect on some superabsorbent properties. The uncrosslinked, so-called soluble, fractions may be increased, for example.
If, however, the reaction takes place too slowly, for example as a result of flawed initiation or incorrect temperature management there may for example be a significant increase in the so-called residual monomer fractions due to insufficient conversion.
In principle, there are different ways to control the polymerization kinetics. For instance, monomer, initiator and crosslinker composition and concentration can be used to influence the kinetics. Owing to the large effect of polymerization kinetics on product quality there is a continuous demand for ways to control the polymerization kinetics.
The problem addressed by the present invention against this background was specifically that of controlling the kinetics of the polymerization of ethylenically unsaturated acid-functional monomers, which optionally at least in part are present in the form of a salt, in the manufacture of water-absorbing polymeric particles.